[hpsdr] Boat Anchors..PreAmps

[Tom Holmes]

Hi Phil...

Re: "Before starting the design of Mercury I was advised by a well know
mathematician that all the signals in the input bandwidth will add together
and overload the ADC. In which case don't bother trying to build such a
receiver."

Seems to me that same problem would be just as likely with ANY receiver
architecture that runs out of headroom, which I am pretty sure would be all
of them :-).

Tom Holmes, N8ZM
Tipp City, OH
EM79


> -----Original Message-----
> From: hpsdr-bounces at lists.openhpsdr.org [mailto:hpsdr-
> bounces at lists.openhpsdr.org] On Behalf Of Phil Harman
> Sent: Friday, April 08, 2011 1:32 AM
> To: lstoskopf at cox.net
> Cc: hpsdr-request at lists.openhpsdr.org; hpsdr at lists.openhpsdr.org
> Subject: Re: [hpsdr] Boat Anchors..PreAmps
> 
> ***** High Performance Software Defined Radio Discussion List *****
> 
> The ADC is a voltage driven device. It will overload (gracefully in the
> case of Mercury) if the instantaneous vector sum of all the signals at its
> input (i.e. after any filtering) exceeds a specified value.
> 
> Before starting the design of Mercury I was advised by a well know
> mathematician that all the signals in the input bandwidth will add
> together and overload the ADC. In which case don't bother trying to build
> such a receiver.
> 
> I also asked Bill Sabin - he agreed with the analysis. But added that this
> would happen once, for a second, in my life time.
> 
> The rest is history...
> 
> 73 Phil...VK6APH
> 
> 
> 
> 
> 
> 
> 
> > ***** High Performance Software Defined Radio Discussion List *****
> >
> > If you look at OLD navy receivers you will often find two stages of RF
> > amps before the mixer with a complex RF filter system.  More than weak
> > signals would require in the bands then used or need for strong signal
> > rejection.  Today's ships do have real problems with multiple signals
> > though.  A lot of that stuff was to prevent LO leakage which might give
> > the ship's position away.  The late W0CY here had several of those and
> > they were magnificent mechanical devices with lots and lots of shielding
> > and DC bypassing.
> >
> > In the case of our ADC's, a very valid purpose of the pre-amp is to
> > protect the input from excess voltages.  I've blown two on a different
RX
> > using the same ADC (lightning once and a leaky TR relay the other).
> >
> > I'm pretty sure that our ADC is a voltage driven device.  Perhaps
someone
> > can explain whether we are looking at overload from the highest voltage
in
> > the band or total power across the band (input).  Or am I back to the dB
> > power vs dB voltage bit?
> >
> > N0UU
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> 
> 
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